This invention relates generally to the field of light emmitting diodes (LEDs). More specifically, the present invention addresses the change in brightness of LED lighting that can occur with changes in ambient temperature. The present invention provides a means for regulating the brightness of LEDs to automatically compensate for various ambient temperatures so that LEDs can be used in lighting applications that experience significant ambient temperature variation.
Light Emitting Diodes (LEDs) are small colored lights that can be seen in or on electronic equipment, household appliances, toys, signs, and many other places. Red, yellow and green LEDs are common and have been around the longest. Other colors, like turquoise, blue, and pure-green are newer. Today""s LEDs can be found in just about every color of the spectrum, including white. LEDs can also emit infrared and ultraviolet light beyond the visible spectrum.
LEDs are different from ordinary light bulbs in that they do not have a filament to break or burn out. They typically last 100,000 hours or more before they need to be replaced. They generate very little heat and require relatively little power. Consequently, LEDs are well suited for a wide variety of applications. The minimal power needs of LEDs make them ideal for use in battery-operated equipment like telephones, toys, and portable computers. The longevity of LEDs make them especially well suited for signage, Christmas lights and other forms of decorative lighting. Today, banks of LEDs are bright enough to illuminate an entire room and are no longer just a dim glow on a stereo.
Diodes generally are electronic circuit components that only allow current to flow in one direction. LEDs are diodes that have the xe2x80x9cside effectxe2x80x9d of producing light while electric current is flowing through them. In the simplest terms, an LED is made with two different kinds of semiconductor material: one type that has an excess of free electrons roaming around inside the material, and another that has a net positive charge and lacks electrons. When an electron from the first material, the donor, flows as a current across a thin barrier and into the second material, a photon or particle of light is produced.
The color of the light depends on a number of factors, including the type of material that the LED is made with and the material""s quantum bandgap (i.e., how much energy each electron needs in order to cross the barrier). A smaller bandgap that fairly slow electrons can cross gives off infrared or red light, while a large bandgap that is crossed only by fast, high-energy electrons gives off light that has a blue or violet color to it.
The LED is a marvel of modern quantum physics, and LEDs are becoming much more commonly used in every type of application imaginable. The unique features of LEDs make them very attractive to many industries. However, one of the drawbacks of LED technology is that the brightness of an LED that is operated with a fixed current is greatly affected by the ambient temperature. For a circuit with a fixed current, a typical LED will shine brighter in colder temperatures and more dimly in hotter temperatures. This effect is charted in FIG. 1.
FIG. 1 illustrates typical luminous flux versus temperature for an HPWT-xH00 LED Emitter driven at a constant 60 mA of current. As shown in FIG. 1, the normalized light output (i.e., brightness) declines steadily as the ambient temperature rises. Specifically, as the temperature changes from xe2x88x9240xc2x0 C. to 85xc2x0 C., the normalized light output changes roughly from 1.74 to 0.52. In other words, when the temperature increases from xe2x88x9240xc2x0 C. to 85xc2x0 C., the brightness decreases by a factor of 3.3.
To illustrate the problem, consider the automobile industry. LEDs are becoming much more widely used in vehicle signal lighting, such as for turning signal lights, stop lights, tail lights, etc. During the night when there is very little light, a turn signal with relatively low brightness may be adequate due to the low light levels. In other words, it is easier to see an LED or any other light at night when little ambient light is present. However, the LEDs that make up a turn signal will likely be relatively brighter at night due to a low ambient nighttime temperature.
On the contrary, during a hot summer day at noon, strong sunlight shoots directly into and around an LED assembly. Consequently, a strong brightness is required for the LED assembly to be visible in spite of the bright ambient glare of the sunlight. Unfortunately, the LEDs may be dimmest under those conditions due to the high ambient temperature.
Consequently, there is a need in the art for a means and method of compensating for the effects of ambient temperature on the brightness of LED lighting so that LED lighting can be effectively used in automobile and other applications that may experience a significant variation in ambient temperature.
The present invention meets the above-described needs and others. Specifically, the present invention provides a means and method of compensating for the effects of temperature on the brightness of LED lighting so that LED lighting can be effectively used in applications that may experience a significant variation in ambient temperature.
Additional advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The advantages of the invention may be achieved through the means recited in the attached claims.
The present invention may be embodied and described as a current regulating circuit for connection between a power supply and one or more light-emitting diodes (LEDs). The circuit includes a temperature-sensitive element that responds to ambient temperature; and a regulator, connected to the temperature-sensitive element, for regulating current flow to the LEDs in response to output from the temperature-sensitive element. The current regulating circuit is configured to provide more current to the LEDs when ambient temperature rises and less current to the LEDs when ambient temperature drops so as to compensate for variations in LED brightness that naturally accompany ambient temperature change.
The regulator may be a voltage regulator that is configured to regulate a voltage difference between the power supply and the LEDs. The voltage regulator may regulate the voltage difference in response to a resistance load connected between ground and the voltage regulator. The resistance load may include the temperature-sensitive element. In such as case, the temperature-sensitive element is preferably a positive temperature coefficient component connected to the voltage regulator. The positive temperature coefficient component may be, for example, a thermistor or a silistor with a resistance that varies with ambient temperature.
The resistance load may also include a resistor for adjusting the compensation depth of the current regulating circuit. The resistor may be connected in parallel or in series with the positive temperature coefficient component.
Alternatively, the regulator may be a voltage regulator that is configured to regulate a voltage difference between the power supply and the LEDs in response to a signal applied to an adjustment terminal of the voltage regulator, the temperature-sensitive element being connected to the adjustment terminal. In this embodiment, the temperature-sensitive element may be a diode. The diode is connected between the output of the voltage regulator and the adjustment terminal of the voltage regulator. This circuit may also include a voltage divider connected to the diode and the adjustment terminal of the voltage regulator for adjusting the voltage applied to the adjustment terminal of the voltage regulator by the diode.
The present invention also encompasses the methods inherent in making and operating the circuits described above and similar circuits. For example, the present invention encompasses a method of regulating current flow between a power supply and one or more light-emitting diodes (LEDs) to compensate for variations in LED brightness that accompany ambient temperature change by: sensing ambient temperature; and regulating current flow from the power supply to the LEDs in response to the ambient temperature. As before, more current is provided to the LEDs when ambient temperature rises and less current is provided to the LEDs when ambient temperature drops to compensate for variations in LED brightness that accompany ambient temperature change.